The present invention relates to a path-measuring device for moving machine parts which has an incremental correction-measurement system for the determination of such guidance errors as offset and tilt of the machine part, as well as to a method of determining the zero position and initial values of the guidance errors.
A path-measuring device of this type is described in European Pat. No. A2-0 082 441. The known path-measuring device has a scale on which, in addition to the scale graduation for measuring displacement in the direction of guidance, there is also at least one additional incremental graduation in the form of a plurality of lines which extend over the entire guidance length. These lines are scanned by two photoelectric signal transmitters or reading heads which are staggered in the direction of guidance, the mean value of the signals of the two transmitters serving to determine offset perpendicular to the direction of guidance, and the difference between the signals serving to determine the angle of tilt of the guided part.
Since this measuring device utilizes an incrementally operating system, it is necessary, at least whenever the measuring device is placed in operation and preferably also during operation, to move to reference points at regular intervals, and to use these reference points to initiate the measurement system with absolute values. For incremental-length measurement systems, it it known, e.g., from West German C No. 3,334,400, to provide reference marks in the form of a plurality of stripes on the scale alongside the longitudinal graduations. These reference marks make possible only the fixing of one reference point, such as the zero point of the measurement device in the direction of guidance. To determine initial values of offset perpendicular to the direction of guidance and of tilt or oblique position of the machine part, the known scales provided with such reference marks are not suitable. To initiate an incremental correction measurement system therefore requires additional measures, which involve special difficulties, since, as a rule, it is not readily possible to offset or tilt the machine part by a specific amount prependicular to its direction of guidance.